This invention is related to logarithmic conversion, and in particular to power measurement in a wireless receiver using a device that converts power measurements in a linear scale to a logarithmic scale to reduce the number of bits required to define the dynamic range.
Automatic gain control (AGC) for radio receivers is well known and widely used. In general, AGC is straightforward for systems that receive signals of approximate constant amplitudes. Communication in a wireless data network is packet-by-packet (“packetized”). Furthermore, packets might arrive simultaneously from several radio transmitters, so each set of packets from a particular transmitter requires its own gain setting. Furthermore, a wireless receiver does not know when packets start. Furthermore, the high data rates of newer wireless data networks lead to problems when trying to include AGC in receivers.
Thus there is a need for fast and accurate AGC. Such fast and accurate AGC requires an estimate of the power of a received signal. A radio receiver produces received signals and may be digitized using one or more analog-to-digital converters (ADCs). The instantaneous and average power may be calculated from samples of received signals, and the average power then used to determine the correct gain setting. In some applications, an average power to gain setting lookup table may be used for such gain determination. The average power, however, is usually available in a linear scale. To obtain accurate gain setting over the complete dynamic range may require a lookup table of prohibitive size. Converting the average power to a compressed scale, for example to a logarithmic scale is thus desirable. One example of a logarithmic scale is decibels (dB).
Thus there is a need for a fast and accurate device to convert numbers in a linear scale to numbers in a logarithmic scale, in particular for implementation for converting power measurements in a radio receiver.
Wireless data networks are beginning to be widely implemented. The EEE-802.11 a is one standard for a wireless local area network for data transmission up to 54 Mbits/s (Mbps). Such a high data rate requires accurate and fast AGC.
Radios-on-a-chip and accompanying modulator-demodulators (modems) on a chip are now being promoted by several companies, e.g., Atheros Communications (Sunnyvale, Calif.) which markets its AR5000 chipset, as does the assignee of the present invention. Such chips put complete 5.15-5.35 GHz transceivers on a chip and complete COFDM modems on another chip, and these chipsets need only a few external filters, a transmit/receive switch and a crystal to operate.
Thus there is a need for a fast and accurate device to convert numbers in a linear scale to numbers in a logarithmic scale, in particular for implementation for converting power measurements in a radio receiver and/or radio implemented on a chip. In particular, there is a need for a method and circuit that achieves linear to logarithmic conversion in a manner compliant with the IEEE 802.11a and similar standards.
For more information on the IEEE 802.11 and IEEE 802.11 a standard, see: ANSI/EEE Std 802.11, 1999 Edition (ISO/IEC 8802-11:1999) Local and metropolitan area network—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, and IEEE Std 802.11a-1999 [ISO/IEC 8802-11:1999/Amd 1:2000(E)] (Supplement to IEEE Std 802.11, 1999 Edition) Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band. The standards are available on the Internet at http://www.manta.ieee.org/groups/802/11/.